In processing seismic data obtained from a particular arrangement of seismic source(s) and receiver(s), one of the commonly performed operations to construct an image of the subsurface from the data is called migration. In data migration, reflection events in the seismic data, in particular in time-domain seismic data records, are moved to their correct locations in the seismic image space, which is typically a three-dimensional space (x-y-z or x-y-time) representing a volume of the subsurface, wherein use is made of prior knowledge of the distribution of seismic wave velocity in the subsurface. So, migration is a method of mapping the data from the seismic data domain to the image domain. Migration is an important tool in seismic interpretation for displaying the locations of geological interfaces (events) in the subsurface. Still, migration methods themselves create some degree of noise and/or image distortion or spurious events, such as so-called migration smiles. The latter is particularly pronounced when processing Vertical Seismic Profiling (VSP) data, measured with seismic receivers in a well penetrating the subsurface and seismic sources at or near the surface. In many situations, the migration smiles are suppressed in the final subsurface image after stacking (adding) all the images over sources and/or receivers.
However, in some surveys such as VSP surveys, the number of source-receiver pairs is often too small to suppress all artifacts in the image.
A particularly popular method of performing seismic data migration, because of its computational efficiency, is referred to as Kirchhoff migration. Kirchhoff migration sums values taken from all seismic traces at the total travel time from the source to an image point and from that image point to the receiver (so-called diffracted wave travel time).
In USA patent application publication No. 2005/0135189 A1 is disclosed a method for Kirchhoff migration of seismic data, wherein the full three components of the reflection wavefield are utilized in a dynamic, vector energy mapping method, in order to reduce unwanted data smearing and false mirror images. To this end, for a source-receiver pair and an image point a single ray-path is considered, and a scalar product of the reflection wavefield vector at the recording time and the unit ray vector at the receiver is used as the migration operator. A weighed summation of the migration operators provides a migrated image point.
Another type of seismic data migration is referred to as wave-equation migration. As is pointed out in the paper “Which depth imaging method should you use? A road map through the maze of possibilities” by D. Bevc and B. Biondi, The Leading Edge, June 2005, pp. 602-606, wave-equation migration has certain advantages over Kirchhoff migration, but is much more demanding in terms of computer power. Wave-equation migration treats seismic waves as the solution of the wave equation, thereby allowing for taking more complex phenomena into account than ray tracing.
Spurious events are a serious problem for image interpretation, in particular in sub-salt geological environments. Due to the high seismic velocity contrast, salt bodies may scatter or focus seismic energy like a lens, thereby reducing the number of source-receiver pairs that contribute to the image of the true reflectors after migration. Kirchhoff migration methods considering a single ray path between each source/receiver pair have shortcomings in this situation. For example, the method known from US 2005/0135189 A1 requires a single unit ray vector at the receiver. However there would be several such vectors in the case of multi-pathing of seismic wave energy, which is typical for a sub-salt environment, and it would not be clear which ray path is to be taken.
UK patent application GB 2321968 discloses a method of processing data obtained from seismic prospecting of an earth formation comprising obtaining data representing a multicomponent seismic signal from a multicomponent seismic receiver at a receiver location in an earth formation, in response to transmitting seismic waves into the earth formation, and performing a migration, based on the Kirchhoff transformation (ray tracing), of the multicomponent seismic signal to obtain a seismic image of the earth formation, wherein the polarization of the seismic wave at the receiver location is taken into account.
The method according to the preamble of claim 1 is known from International patent application WO 2004/067912.
There is a need for a seismic data migration method that operates well in a complex situation where points in the subsurface can be connected by multiple ray paths (i.e. multi-pathing) and/or in when diffraction (forward scattering) of seismic waves occurs, such as in subsalt, and where spurious events such as migration smiles can be suppressed.